The present invention relates to a cleaning device for cleaning a surface of a body by intermittently feeding a cleaning belt in compressive contact with the surface. The invention also relates to a fixing device including a fixing roller as a body to be cleaned. The invention further relates to an image forming apparatus including such a fixing device.
An image forming apparatus for electrophotographic image formation includes a fixing device with a pair of fixing rollers. One of the fixing rollers is a heating roller heated to a temperature at which toner can melt. The other fixing roller is a pressing roller in compressive contact with the heating roller. While a recording medium such as paper having a toner image transferred to it is passing between the fixing rollers, the fixing device heats and presses the medium so as to fix the image on the medium.
While the recording medium is passing between the fixing rollers, part of the toner transferred to the medium sticks to the cylindrical surfaces of the rollers. The toner on the roller surfaces would dirty or spoil the following recording media passing between the fixing rollers. Therefore, the fixing device is fitted with cleaning devices for removing the toner on the roller surfaces.
In recent years, it has been demanded that image forming apparatus perform image formation at higher speed and accordingly feed recording media at higher speed. When recording media pass at higher speed between the fixing rollers of an image forming apparatus, larger amounts of toner stick to the cylindrical surfaces of the rollers. The cleaning devices fitted to the fixing device of the apparatus need to remove large amounts of toner on the roller surfaces reliably over a long time.
As disclosed in JP-2003-107952A, a conventional cleaning device includes a cleaning belt called a web sheet, which may be made of woven cloth. The cleaning belt is wound on an unwinder and can be fed intermittently from it via a cleaning position, where the belt is brought into compressive contact with the cylindrical surface of a fixing roller. The fed belt is then wound up by a winder. The cleaning belt is impregnated with a cleaning agent such as silicon oil.
Such a cleaning device can also be applied as a device for cleaning a dirty surface of a part of a device other than the fixing device of image forming apparatus.
As the cleaning belt is wound up repeatedly, its portion remaining on the unwinder decreases, so that a trailing end portion of it becomes unable to be held reliably by the unwinder. The unwinder may include a delivery roller, the cylindrical surface of which is wound with the cleaning belt. If the portion of the cleaning belt that remains on the delivery roller becomes shorter than the circumference of the roller, no sufficient tension can be applied to the belt, so that the belt cannot come into compressive contact with the fixing roller in the cleaning position. This results in insufficient cleaning. If the belt portion on the delivery roller becomes shorter than the roller circumference, a trailing end portion of the cleaning belt becomes loose. The loose belt portion may come into contact with other parts of the apparatus than the fixing roller, so that the parts may be dirtied with the cleaning agent with which the cleaning belt is impregnated, and/or their functions may lower.
Another conventional cleaning device counts the rotations of the delivery roller of an unwinder or the wind-up roller of a winder. Based on the number of rotations of the roller, the cleaning device determines the length of the portion of a cleaning belt that is wound on the roller. Before a trailing end portion of the cleaning belt becomes unable to be held reliably on the unwinder, the user is prompted to replace the belt.
However, the relation between the number of rotations of the delivery or wind-up roller and the length of the belt portion wound on the roller varies with the number of turns of the wound belt portion. Besides, because the cleaning belt is flexible, the relation between the number of rotations and the length of the wound belt portion is not always equal even if the number of turns is equal. In particular, because the wind-up roller is wound with a portion of the cleaning belt to which toner has stuck, it is impossible to accurately determine the relation between the number of rotations of this roller and the length of the belt portion wound on the roller.
Accordingly, it is impossible to accurately detect the length of the wound portion of the cleaning belt of a conventional cleaning device. This prevents the user from being prompted to replace the cleaning belt at the proper time. If the belt replacement is delayed, the body to be cleaned cannot be cleaned sufficiently, and the cleaning agent may dirty other bodies and lower their functions. If the belt replacement is too early, the cleaning belt is thrown away with a long portion of it remaining unused. This results in a high running cost.